(435h) Microbead Arrays Formed by Flow Entrapment in a Microfluidic Cell and the Effect of the Trap Geometry On the Mass Transfer of Targets to Probe Molecules On the Particle Surfaces
Platforms which display biomolecules in an array for the multiplexed screening of the binding interactions of these molecules with molecular targets are important tools in biomedical research (drug discovery), clinical diagnostics ( biomarker identification) and sensor development. Platforms which integrate the array and the binding detection in a single, microfluidic format have the advantage of reduced volumes of reagents and targets, high sensitivity, and increased throughput. Microbead-based arrays are biomolecular displays in which the probe molecules are first linked to micron-sized beads which are arranged and fixed to a surface. These arrays can easily be constructed in a microfluidic platform by using a concourse of traps designed to hydrodynamically capture the beads as they stream through the course. The arrangement of the traps serves as the template for the microbead array. By trapping the beads in a sequence of stages, in which at each stage, beads displaying only one probe are trapped, with their entrapment locations mapped, a fully indexed array can be formed. The array can then be screened against probes which are passed through the concourse.
The traps used to capture the microbeads have to properly designed to not only efficiently capture the microbeads, but also to minimize, during the assaying step, diffusion limitations in the transport of the target molecules to the surface of the captured particle. Eliminating diffusion barriers increases the dynamic response of the array, which is then only limited by the kinetics of the surface binding of the target to the probe. This presentation will examine experimentally and theoretically how trap geometries can be designed to reduce diffusion barriers. We study a general trap design consisting of an open cavity or chamber, facing the flow, in which the particle is captured within the chamber. The traps are placed in a staggered arrangement. To illustrate the effect of the trap shape on the mass transfer of the target, we examine the binding of the ligand biotin attached as the probe to the surface of the microbead, Neutravidan, which binds selectively to the biotin, is streamed through the cell. By fluorescently labeling the Neutravidin, and recording the accumulating fluorescence at each bead location by using confocal laser scanning fluorescence microscopy and epifluorescent microscopy, the rate of conjugation of the protein to the displayed ligand is measured as a function of time. Traps with large cavities sequestering beads deeply in the enclosing chamber are found to have significant diffusion barriers relative to traps in which the microbeads are held in smaller cavities. The capture efficiency is reduced by the smaller capture area of these traps, but we demonstrate that hydrodynamic arraying is still effective. Numerical solutions of the convective diffusion equations are presented for the diffusion and binding of the protein to the biotin surface of the microbeads for different trap geometries. These simulations confirm the experimental results, and provide an accurate measurement of the kinetic constants for the attachment of the protein to the biotin.